On 12/08/2010 11:13, Johan Tibell wrote:
There needs to be some amount of code generation, but much of the
implementation can still be shared. I previously tried to defined the
type class as
{-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}
module Ex2 where
import Prelude hiding (lookup)
data MapView k v = TipView
| BinView {-# UNPACK #-} !Size !k !v !(Map k v)
!(Map k v)
class Unbox k v where
data Map k v :: *
tip :: Map k v
bin :: Size -> k -> v -> Map k v -> Map k v -> Map k v
view :: Map k v -> MapView k v
type Size = Int
lookup :: (Ord k, Unbox k v) => k -> Map k v -> Maybe v
lookup k m = case view m of
TipView -> Nothing
BinView _ kx x l r -> case compare k kx of
LT -> lookup k l
GT -> lookup k r
EQ -> Just x
{-# INLINE lookup #-}
Calling lookup from a different module at a know type gives exactly the
Core you'd like to see (unpacked types, no MapView constructors).
I'm not sure I want lookup (and other operations) to be inlined at every
call site though.
Rather than try to solve this problem in one go, I would go for a
low-tech approach for now: write a TH library to generate the code,
and ask the user to declare the versions they need. To make a
particular version, the user would say something like
module MapIntDouble (module MapIntDouble) where
import TibbeMagicMapGenerator
make_me_a_map ...
there's no type class of course, so you can't write functions that
work over all specialised Maps. But this at least lets you generate
optimised maps for only a little boilerplate, and get the
performance boost you were after.
This doesn't quite work though as two MapIntDouble defined in two
different libraries are incompatible. This is essentially the same
problem as with instance collisions.
But you get to choose the module name, so you can avoid collisions by
using qualified names.
Cheers,
Simon
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